The present invention relates to extrusion machinery for thermoplastics and particularly to the design of the rotor used in the extruding machinery. In the extrusion of plastics, at times it is necessary to mix a low viscosity or immiscible additive with a high viscosity polymer melt. The low viscosity or immiscible ingredient may be a coloring agent, an ingredient used to improve performance of the polymer product, or a material which facilitates subsequent processing of the polymer. Volatile materials introduced in order to cool the polymer by evaporation at a downstream location in the extruder exemplify this latter class of ingredients. It is, of course, required that the low viscosity ingredient be intimately mixed with the polymer melt prior to extrusion to provide a uniform extrusion. In addition, it is frequently important that the mixing be accomplished with minimal dissipation of energy in the polymer and in as short a time as possible in order to reduce thermal and mechanical damage to the polymer.
Mixing a low viscosity fluid into a high viscosity polymer melt is not a simple task. Karam and Bellinger I&EC Fundamentals, Vol. 7, No. 8, pp. 576-581, 1968, have shown experimentally that breakup of droplets into smaller droplets - as required for intimate mixing - is very unlikely to occur when the viscosity of the dispersed phase is 0.005 or less of the viscosity of the bulk phase. Numerical calculations have confirmed, in the words of Rallison, "The Deformation of Small Viscous Drops and Bubbles in Shear Flows, Annual Review of Fluid Mechanics, 1984, Vol. 16, pp 45-66, that "low viscosity drops can attain highly extended stable shapes and require very strong flows to break them."
One possible solution to the mixing of low viscosity or immiscible ingredients with a high viscosity polymer melt is discussed in an article by T. Sakai entitled "Observations on the Flow Behavior in Twin Screw Extruders with Mixing Elements" appearing in Vol. 38, No. 4, pp 279-284, 1981, of Kobunshi Ronbunshu. In this article there is discussed the performance of various types of rotors used in extruding elements, together with observations of mixing behavior vis a vis additives of viscosity comparable to that of the polymeric fluid. One of the rotors shown comprises a flighted rotor having a section in the center in which the forward flights are removed and replaced by radial mixing pins described in the article. This type of rotor has been used in both single screw and twin screw extruders. Other mixing elements of similar design have been described by C. Y. Cheng, "Extruder Screw Design for Compounding", Plastics Compounding, Vol. 4, No. (2), March-April 1981, pp 29-40, and by B. Gallagher and A. G. M. Last, "Process for Improving the Dispersion of Blowing Agents in Thermoplastics", United Kingdom Pat. No. 1,410,979.
While the rotor elements having mixing pins as described in the above articles can provide some mixing of a low viscosity or immiscible additive with a high viscosity polymer melt, they do so at the price of very high energy dissipation in the melt. In addition, because the elements described have little or no forwarding capability, the polymer channels in the elements are completely filled with polymer at all times. This increases the residence time of the melt in the mixing stage as well as leading to a requirement that upstream stages of the extrusion machinery generate sufficient pressure to cause the melt to flow through the mixing stage. Both factors increase the potential for mechanical and thermal damage to the polymer. Moreover, complete fillage of the polymer channels leads to reduced mixing efficacy as described below.